A steel bar is generally cut from a bar material in accordance with a consumer's purchase order (hereinafter referred to as "a material adoption"). Thus, a loss of material is produced in the cutting step. A further material loss occurs when material adoption is conducted by dividing a bar material into a plurality of bars, aligning the several divided bar materials in parallel and then cutting them into the necessary length.
Heretofore, an optimum cutting control of a steel bar has been proposed and disclosed, for example, in a thesis by K. Inasaki et al entitled "Improvements in Cold Shear Yield of Bar Mill by Computer" Control System, pages 207 to 213 of "Tetsu-to-Hagane" Vol. 67, No. 15, in 1981. According to this proposal, a rolled material is divided into a plurality of bar materials, and the number of times to repeatedly cut the divided bar materials into desirable lengths is determined so as to minimize material loss and, hence, optimize the overall operation.
In general, this cutting control relies on the principle of optimization which can be described by the equation: ##EQU1## where g.sub.k (x): the loss when the k-th divided bar material is cut x times for the material adoption
f.sub.k (x): the minimum loss expected when k pieces of divided bar materials are cut for the material adoption PA0 f.sub.1 (x)=g.sub.1 (x) PA0 k=2 to N PA0 N: The number of divided bar materials PA0 X: Total number of bars to be cut in parallel PA0 M.sub.k : The maximum number of cuts possible from the divided bar material
In other words, since the f.sub.k-1 (X-x) is the minimum loss when (k-1) pieces of the divided bar materials are cut for (X-x) times of material adoptions, when the x is determined so that the total sum of the f.sub.k-1 (X-x) and the loss g.sub.k (x) when the k-th divided material is cut x times may become minimum, it coincides with f.sub.k (x). If the maximum possible number of cutting the k-th divided material is represented by M.sub.k, the value of x can take an integer numbers from 0 to M.sub.k.
Though the conventional cutting control system is executed as described above, the prediction of the length of the rolled bar material causes an error in the actual rolling length due to a cutting operation based the prediction of the length according to the weight of the material. Further, since cutting errors also take place at the respective cutting position of a flying shear, a considerable difference in length occurs and material loss drastically increases. Thus, it is difficult to minimize the loss of the bar materials.